Nomogram for estimating specific consanguinity risks.

Consanguinity is an important problem in medical genetics because it significantly influences morbidity risks for recessive disorders. Genetic counsellors manage this problem in very simple terms according to Mendelian rules but without considering the theory of inbreeding in full. Consequently, the severity of specific recessive genetic risks is not accurately estimated. The purpose of this paper is to set out a nomogram to solve some consanguinity problems in cases of autosomal recessive disorders. A nomogram is a graphical representation of numerical relations that enables the value of a dependent variable to be read off when the value of two or more independent variables are given. The figure shows a nomogram plotted on a logarithmic scale depicting the classical variables of the theory of inbreeding according to the equations given by Li.12 This nomogram presents the frequency of inbred homozygous recessive subjects (RF = q2+Fpq) as a function of the ratio of inbred homozygous (RF) to panmictic homozygous subjects (Ro = q2) for different values of inbreeding coefficients (F) and gene frequencies (q) of recessive alleles. The relative consanguinity risk defined as q2+Fpq/q2 may be computed also as 1 +F/q when q is small. This nomogram illustrates in full the three basic properties ofa system ofinbreeding that maybeuseful in accurately estimating consanguinity risks in man. (1) Coefficient F measures the degree of increase of homozygosity and decrease of heterozygosity, and consequently represents the degree of fixation of the recessive allele q. The lines ofF are plotted converging to the point of fixation, where all the variables merge (F,q,RF = 1). (2) In a system of inbreeding the gene frequencies (q) are independent of the inbreeding coefficient (F). The nomogram shows that increasing F does not change gene frequencies, which are plotted perpendicularly to the F lines. (3) The proportion of recessives may be divided